270 



PRINCIPLES OF ELECTRICAL DESIGN 



that the air gap near the pole tip would be approximately as 

 determined by the method here described. 



With the cylindrical field magnet it is not usual to shape the 

 pole face. The clearance between the tops of the teeth on 

 armature and rotor would have a constant value, the proper 

 distribution of flux over the armature surface being obtained 

 by spreading the field coils over the periphery of the cylindrical 

 rotor. From 15 to 25 per cent, of the pole pitch is left unwound 

 at the center of the pole. This unwound portion is usually 

 slotted, but it can be left solid if it is desired to reduce the re- 



Alternative Three- Part 



Wedge, which allows of 



Central Wedge being 



pressed down while the 



Steel Liners are driven in 



Detail of Slot and 

 Wedge 



FIG. 103. Rotor of four-pole turbo-alternator, with radial slots. 



luctance of the air gap at the center of the pole face while yet 

 retaining the cylindrical form of rotor. One feature of equally 

 spaced slots over the surface of the rotor is that there are no 

 sudden changes in the air-gap permeance the average value of 

 which is then the same at all points on the periphery. An Argu- 

 ment in favor of slotting the unwound portion of the pole face 

 is that the field may be " stiffened " by using high flux densities 

 in the teeth. 



Fig. 103 is a section through part of the rotor of a four-pole 

 turbo-alternator. The rotor of a two-pole machine may be 

 constructed in the same way, with radial slots, but parallel 



